Der Einfluß von chemischer Konstitution, Symmetrie und molekularer Umgebung auf die intramolekulare Schwingungsrelaxation aromatischer Moleküle

Abstract

This thesis focuses on elucidating mechanisms of intramolecular vibrational energy redistribution (IVR). It constitutes a comparative study on structurally closely related aromatic molecules in different molecular environments. Generally, from a chemist`s point of view it is crucial to understand vibrational energy transfer phenomena in solution. However, in the solution phase a competition between intra- and intermolecular vibrational energy transfer (VET) exists. It is therefore of great advantage to first study quasi-isolated molecules in the gas phase, where the timescales of IVR and VET are well seperated. The next step then is to use supercritical solvents and to gradually increase the influence of the molecular environment by stepwise increasing their density.The experimental technique employed in this work was femtosecond IR pump UV probe spectroscopy. After local excitation of high frequency CH-stretch vibrations the progress of vibrational energy redistribution was monitored by measuring the energy content of low frequency Franck-Condon-active normal modes of the molecule. In the context of this thesis it could for the first time be demonstrated that this method is perfectly suited to study a certain model system in different phases.Gas phase experiments clearly showed that the intramolecular redistribution of the excitation energy obeys a hierarchical coupling mechanism. The first redistribution step occurs on a sub-ps timescale. In case of the second step time constants ranging from 48 ± 5 ps (benzene) to 3.8 ± 0.5 ps (trifluorotoluene) were determined. There exists no correlation between rate of energy redistribution and total density of states of the aromatic molecules.The influence of chemical substitution on IVR was studied with special emphasis on molecular symmetry and internal rotation of selected benzene derivates. A comparison of benzene and benzene-d1 revealed an three- to fourfold acceleration of IVR through monodeuteration. This trend could be exactly reproduced in computer simulations of the intramolecular energy flow in vibrational quantum number space. Based on these findings the relative influence of molecular symmetry and vibrational state space anisotropy on IVR was assessed. For toluene and trifluorotoluene it was shown that in the case of phenyl CH-stretch excitation an internal rotor accelerates IVR much less efficiently than when low frequency ring modes are initially populated. A comparison of relative substituent effects as well as low order coupling modelling gave clear evidence that the substituent`s internal degrees of freedom are distinctively decoupled from the vibrational dynamic of the phenyl moiety whenever high frequency phenyl-CH-stretch modes serve as the excitation chromophore.Experiments in the supercritical phase revealed an acceleration of the slow IVR component with increasing solvent density. With the help of Monte-Carlo-calculations a direct correlation between IVR rate and the frequency of solute-solvent collisions was established. By varying the molecular environment it became clear that the average energy transferred per collision has only a minor influence on IVR dynamics. Furthermore it was shown that in the condensed phase due to the solvent induced contraction of the slow IVR timescale different molecules exhibit strikingly similar dynamics. Thus the substituent effects observed in the gas phase experiments can be considered as "masked" in solution.